1. Field of the Invention
The present invention relates to shock absorbers and, more particularly, to shock struts for use with aircraft landing gear having a hydraulic chamber combined with a pair of gas chambers operating in series during a first range of energy absorbtion and connected in parallel during a second range of energy absorbtion loads.
2. Description of the Prior Art
The primary purpose of landing gear on an aircraft is to support the aircraft on the ground during normal operations of taxiing, takeoffs and landings. A secondary purpose of such landing gear is to absorb energy in the event of a hard landing and to enhance survival of the occupants of the aircraft in the event of a very hard landing, all without affecting the energy absorbtion capability during normal aircraft operations. Conventional shock absorbing struts, which are part of the landing gear of aircraft limit the loads transferred to the aircraft by absorbing energy through forcing oil flow from an oil chamber to compress a gas chamber as a function of the extent of stroking of the strut. As the strut strokes, the gas chamber telescopes into the oil chamber to force oil through an orifice into the gas chamber resulting in compression of the gas. Usually, a floating piston maintains segregation between the oil and the gas. Control of the oil flow and the gas pressure is vital to the performance of the shock strut.
During normal operations of an aircraft, such as taxiing, a shock strut should provide a force that supports the aircraft and that acts as a spring to soften bumps due to ground surface variations. A resistance to dampen rebound or bouncing of the aircraft should be incorporated. For normal landings, which may be defined as low landing sink rates in the range of 0 to 15 feet per second after the tires have come into contact with the ground, the landing gear should minimize fatigue loading to the fuselage. For moderately hard landings defined as medium sink rates, the landing gear should absorb energy to fully decelerate the fuselage to preclude ground contact by the fuselage. Moreover, the landing gear should prevent expensive fuselage repair by limiting the applied loads to less than the yield strength of the fuselage components. For crash conditions defined as high sink rates up to 42 feet per second, the primary purpose of landing is to protect the occupants of the aircraft by absorbing as much energy as possible to limit the loads imposed to survivable levels.
One known shock strut for absorbing energy imposed in a high sink rate environment incorporates two stages. The first stage includes an a hydraulic chamber interconnected with a gas chamber under pressure through an orifice and poppet valve. A serially attached second stage includes a similar gas chamber at a substantially higher initial pressure. The purpose of this configuration is to permit the lower pressure gas chamber to absorb a range of energy expected from sink rates experienced during normal landings. The second high pressure gas chamber is intended to absorb energy during hard landings resulting from a much higher range of sink rates. In theory, such an arrangement would seem to be idyllic in absorbing energy and limiting the loads imposed. However, because of the exponential pressure increase within the gas chambers, gas pressure produced loads will often exceed the design limit and result in catastrophic failure of the shock strut.